1. Technical Field
The present invention is directed to the encoding and decoding of Multiplexed Analog Components (MAC) color television signals. More specifically, the present invention is directed to the frequency generation for MAC color television encoding and decoding.
2. Background Information
Composite color television signals, as is well known to those skilled in the art, are those signals wherein the chrominance (i.e., color) information is carried on a subcarrier and is transmitted along with the luminance (i.e., brightness) information. In the United States, color broadcasts are made according to National Television Systems Committee (NTSC) standards. Other composite signals include SECAM, which is used in France, and PAL, which predominates in the rest of Europe.
FIG. 1 shows a radio frequency amplitude vs. frequency diagram where the NTSC signal amplitude modulates an RF carrier and is filtered to vestigal sideband amplitude modulation as for terrestrial broadcasting under FCC rules. A typical NTSC composite color television signal is shown, and includes luminance signal 12 and chrominance signal 14. The signal occupies a nominal radio frequency bandwidth of 6 MHz with picture carrier 16 being 1.25 MHz above the lower end of the band. Luminance information is amplitude modulated directly onto the picture carrier. Chrominance information is modulated onto color subcarrier 18 which is, in turn, used to modulate the picture carrier, while audio information is modulated on subcarrier 20. The color subcarrier has a frequency of 3.579545 MHz, a standard established by the NTSC. The lower chrominance sideband and portions of the lower sideband of luminance are suppressed, not transmitted.
It has long been recognized that composite color television signals have problems associated with their structure. For example, the overlap of chrominance and luminance information in region A leads to problems since separation of luminance and chrominance is accomplished by filtering the frequency-division multiplexed signal. If, upon reception, complete separation between luminance and chrominance is desired, the necessary filtering will cause the loss of some of the information can be tolerated, then one must accept interference between the luminance and chrominance signals. Moreover, since the various parts of the NTSC television signal are transmitted at different frequencies, non-uniform amplitude/frequency response occurring in transmission will affect them differently, causing the signal to deteriorate. Also, the available color information is severely limited by the small color bandwidth permitted by the choice of color subcarrier.
The Multiplexed Analog Components (MAC) color television signal was developed to overcome the problems inherent with composite color television systems. Turning now to FIG. 2, an amplitude-vs.-time diagram of a MAC video line is shown, and includes horizontal blanking interval (HBI) 22, chrominance information 24 and luminance information 26 (either or both of which may be time-compressed), the latter being separated by guard interval 28 to assist in preventing interference between the two signals.
The MAC color television signal is obtained by generating conventional luminance and chrominance signals (as would be done to obtain a conventional NTSC or other composite color television signal) and them sampling and storing these signals separately. Luminance is sampled at a luminance sampling frequency and stored in a luminance store, while chrominance is sampled at a chrominance sampling frequency and stored in a chrominance store. The luminance and chrominance samples may then be compressed in time by first writing them into the store in their individual sampling frequency and reading them from the store at a higher frequency. A multiplexer selects either the luminance store or the chrominance store, at the appropriate time during the active video line, for reading, thus creating the MAC signal of FIG. 2. If desired, audio samples may be transmitted during the HBI; these are multiplexed (and may be compressed) in the same manner as the video samples. The single rate at which all samples occur in the multiplexed MAC signal is called the MAC sampling frequency.
Different physical embodiments have been developed which implement the MAC format of FIG. 2. For example, several MAC formats have been realized with a 3:2 luminance compression. European C,D and D/2 for 625 lines per frame system utilize a MAC sampling frequency of 20.250 MHz. B-MAC for both 625 and 525 lines per frame systems employ a MAC sampling frequency of 1365 f.sub.H, where f.sub.H is the line scanning frequency. A further embodiment uses a MAC sampling frequency of 14.32 MHz with a 4:3 luminance compression and still another embodiment uses a MAC sampling frequency of 21.447 MHz with a 5:4 luminance compression.
Drawbacks to all of the above MAC embodiments include both limited luminance and chrominance resolution as well as complexity in the generation of the various clock frequencies in receivers. These drawbacks have been resolved in the physical embodiment shown below with reference to Table 1.
TABLE 1 ______________________________________ Fraction of Signal Frequency Master Clock ______________________________________ Master Clock (f.sub.0) 42.95 MHz = 2730 f.sub.H 1 Luminance 14.32 MHz = 910 f.sub.H 1/3 Sampling (f.sub.1) Chrominance 7.16 MHz = 455 f.sub.H 1/6 Sampling (f.sub.2) Audio 0.33 MHz = 21 f.sub.H 1/130 Sampling (f.sub.3) MAC (f.sub.4) 21.48 MHz = 1365 f.sub.H 1/2 Sampling Teletext 6.14 MHz = 390 f.sub.H 1/7 Generator (f.sub.5) NTSC Color 3.579545 MHz = 227.5 f.sub.H 1/12 Subcarrier ______________________________________ (Frequency f.sub.3 may also be 0.20 MHz, or 13 f.sub.H, which is 1/210 of f.sub.0.)
The system incorporating the embodiment shown in Table 1 is discussed in detail in application Ser. No. 652,926 filed Sept. 21, 1984 now U.S. Pat. No. 4,652,903, issued to K. Lucas. The Lucas patent is commonly assigned to the assignee of the present invention, and is herein incorporated by reference.
The Lucas embodiment incorporates frequencies (to be used as sampling frequencies and for other purposes) related to each other such that they can be derived from a single master clock frequency, simply by division by integer values. Thus, as no frequency multiplication is involved (which would be required if a selected frequency was not divisible by an integer into the master clock frequency), only a single phase-locked loop is required. This feature simplifies and reduces the cost of the equipment required at the receiver.
The limitation on the upper frequency of information which can be carried without distortion due to aliasing is one-half (50%) the sampling frequency (i.e., the Nyquist rate). The requirement for an economically realizable set of filters in the receiver further reduces the useful bandwidth to approximately 40% of the sampling frequency. Thus, for the family of frequencies shown in Table 1, the luminance frequency response is limited to approximately 6 MHz, while the chrominance frequency response is limited to approximately 3 MHz.
Two of the aforementioned MAC embodiments have been reduced to commercial practice are described in documents to be published by the CCIR, an international body which studies the standardization of television signals for international program exchange. These embodiments are the 525 line B-MAC and 625 line B-MAC. C and D/2 MAC will soon be used in Europe for Direct Broadcast (via) Satellite. Receivers are presently in use in several countries for transmissions within the B-MAC standards and use in other countries is expected shortly. Because of the luminance sampling frequencies employed within these standards, these transmissions have inherently limited resolution as noted above. There is increasing interest in improved resolution with widescreen pictures and it is highly desirable to extend the performance of these MAC systems, as discussed above, to provide increased resolution for new receivers while retaining full compathbility with existing MAC receivers for an orderly transition to improved television (HDTV). These MAC embodiments already provide for both widescreen and standard aspect ratio picture transmission and display.